diff --git a/openbis/source/java/ch/systemsx/cisd/openbis/generic/client/web/server/calculator/GridExpressionUtils.java b/openbis/source/java/ch/systemsx/cisd/openbis/generic/client/web/server/calculator/GridExpressionUtils.java
index b0d24305aba1cb6e0fc6f208d14f0fdc15d02d33..edf0eebd6333aaee4eb0d1cb15aeb7aa2d8b916c 100644
--- a/openbis/source/java/ch/systemsx/cisd/openbis/generic/client/web/server/calculator/GridExpressionUtils.java
+++ b/openbis/source/java/ch/systemsx/cisd/openbis/generic/client/web/server/calculator/GridExpressionUtils.java
@@ -129,7 +129,7 @@ public class GridExpressionUtils
                 RowCalculator<T> calculator = calculators.get(columnId);
                 PrimitiveValue value = evalCustomColumn(rowData, customColumn, calculator);
                 // don't modify type if it is a null string
-                if (value.toString() == null)
+                if (value.toString() != null)
                 {
                     customColumn.setDataType(DataTypeUtils.getCompatibleDataType(customColumn
                             .getDataType(), value.getDataType()));